Aluminum tank, yes? It all depends on the nature of the dent. First, if there is any indication of a crease or kink forget trying. If the dent is a simple shallow belly, a lot can be done with air pressure, heat and light hammering around the perimeter of the dent. Go easy on the pressure, just few PSI, or you will turn that tank into a beach ball. Someone here likely has more info or even a video tutorial. If you had been a Constantine last summer, Jay put on a very good seminar and demonstration on just this thing. ALSO,, the old Johnson service manual has instructions.
Joe B